Scanning Systems

ABSTRACT

The invention provides a method and system for scanning an object comprising providing a first detector region having a thickness of at least 2 mm and a second detector region having a thickness of at least 5 mm wherein the second detector region is arranged to receive radiation that has passed through the first detector region. The method comprises irradiating the object with radiation having a peak energy of at least 1 MeV, and detecting the first profile radiation after it has interacted with or passed through the object in order to provide information relating to the object. Detecting the first profile radiation comprises detecting the first profile radiation at the first detector region, receiving the first profile radiation that has passed through the first detector region at the second detector region, and detecting the first profile radiation at the second detector region. The scanning method further comprises irradiating the object with radiation having a second energy profile, relatively lower than the first energy profile, and having a peak energy of at least 0.5 MeV, detecting the second profile radiation after it has interacted with or passed through the object in order to provide information relating to the object. Detecting the second profile radiation comprises detecting the second profile radiation at the first detector region, receiving the second profile radiation that has passed through the first detector region at the second detector region, and detecting the second profile radiation at the second detector region.

CROSS REFERENCE

The present application is a national stage application ofPCT/GB2009/000493, filed on Feb. 25, 2009, which further relies on GreatBritain Patent Application Number 0803641.0, filed on Feb. 28, 2008, forpriority.

FIELD OF THE INVENTION

The present invention relates to scanning systems. It has particularapplication in scanning systems for cargo, but can also be used inscanners for other applications such as security and high energy medicalscanners.

BACKGROUND OF THE INVENTION

There is a requirement for screening of cargo for the purpose ofidentifying the presence of illicit materials and objects. Currently,such screening is often performed using X-ray scanners.

X-ray scanners for cargo inspection typically comprise a high energyX-ray source (usually based on an X-ray linear accelerator) with a beamquality of 4 MeV to 9 MeV. The X-ray output from the X-ray linearaccelerator is then collimated down to a narrow fan-beam of radiationwhich is shone through the item of cargo under inspection. A lineararray of X-ray detector elements is then positioned opposite to theX-ray source such that it is irradiated by the fan-beam of radiationafter attenuation of the X-ray beam by the object under inspection.

SUMMARY OF THE INVENTION

An aspect of the invention comprises a scanning method for scanning anobject comprising: providing a first detector region and a seconddetector region wherein the second detector region is arranged toreceive radiation that has passed through the first detector region;irradiating the object with radiation having a first energy profile;detecting the first profile radiation after it has interacted with orpassed through the object in order to provide information relating tothe object, wherein detecting the first profile radiation comprises:detecting the first profile radiation at the first detector region;receiving the first profile radiation that has passed through the firstdetector region at the second detector region; detecting the firstprofile radiation at the second detector region; the scanning methodfurther comprising: irradiating the object with radiation having asecond energy profile, different to the first energy profile; detectingthe second profile radiation after it has interacted with or passedthrough the object in order to provide information relating to theobject, wherein detecting the second profile radiation comprises:detecting the second profile radiation at the first detector region;receiving the second profile radiation that has passed through the firstdetector region at the second detector region; detecting the secondprofile radiation at the second detector region.

In one embodiment, the present invention is a scanning method forscanning an object comprising: providing a first detector region havinga thickness of at least 2 mm and a second detector region having athickness of at least 5 mm wherein the second detector region isarranged to receive radiation that has passed through the first detectorregion; irradiating the object with radiation having a peak energy of atleast 1 MeV; detecting the first profile radiation after it hasinteracted with or passed through the object in order to provideinformation relating to the object, wherein detecting the first profileradiation comprises a) detecting the first profile radiation at thefirst detector region, b) receiving the first profile radiation that haspassed through the first detector region at the second detector region,and c) detecting the first profile radiation at the second detectorregion; the scanning method further comprising: x) irradiating theobject with radiation having a second energy profile, relatively lowerthan the first energy profile, and having a peak energy of at least 0.5MeV and y) detecting the second profile radiation after it hasinteracted with or passed through the object in order to provideinformation relating to the object, wherein detecting the second profileradiation comprises: i) detecting the second profile radiation at thefirst detector region; ii) receiving the second profile radiation thathas passed through the first detector region at the second detectorregion; and iii) detecting the second profile radiation at the seconddetector region. Optionally, the method comprises positioning the firstdetector region between the object and the second detector region. Themethod comprises determining information relating to the object basedupon information from the first and second detector regions relating tothe first and second energy profile radiation. The method comprisesdetermining information by inputting the information from the first andsecond detector regions relating to the first and second energy profileradiation into a least squares minimization technique to obtaininformation relating to the object.

The method comprises calculating the ratio, (A/B)₁/(A/B)₂ in order tothe determine information relating to the object based upon thecalculated ratio, wherein A is indicative of the amount of radiationdetected at the first detector region, B is indicative of the amount ofradiation detected at the first detector region, (A/B)₁ is the ratio offirst profile radiation detected at the first detector region relativeto first profile radiation detected at the second detector region, and(A/B)₂ is the ratio of second profile radiation detected at the firstdetector region relative to second profile radiation detected at thesecond detector region.

The method comprises irradiating and detecting the first profileradiation before the second profile radiation, or vice versa, whereinirradiating the object comprises irradiating the object in discretebursts. The method comprises sending detected information received inresponse to a burst from the detector regions before the next burstoccurs.

The low energy profile radiation comprises 3 MeV x-ray radiation and thehigh energy profile radiation comprises 6 MeV x-ray radiation. Themethod comprises configuring the first detector region and the seconddetector region to detect a predetermined amount of radiation relativeto each other.

The method comprises configuring the first detector region and thesecond detector region to detect substantially the same amount ofradiation as each other.

The method comprises configuring any one or more of size, shape ormaterial of each detector region so that the first detector region andthe second detector region detect the predetermined amount of radiationrelative to each other.

The method comprises providing a first detector including the firstdetector region and a second detector including the second detectorregion. The method comprises irradiating the object with radiation atmore than two energy profiles, such as at three energy profiles or fourenergy profiles or five energy profiles or six energy profiles or sevenenergy profiles.

In another embodiment, the present invention is directed toward a methodof scanning overlapping objects comprising using the method of claim 1to determine information relating to each overlapping object in a regionof the object which does not overlap another object and using thedetermined information to calculate a reference detection value orvalues relating to a value or values expected to be detected in theregion in which the objects overlap in the absence of further objectsthat are not present outside the overlapping region and using the methodof any preceding claim to ascertain information relating to the regionin which the objects overlap and comparing the ascertained informationto the expected values to determine whether an additional object ispresent within the region in which the objects overlap.

In another embodiment, the present invention is directed toward ascanning system for scanning an object comprising: a variable energylevel radiation source arranged to irradiate an object with radiationhaving a plurality of different energy profiles including a first energyprofile having a peak energy of at least 1 MeV and a second relativelylower energy profile having a peak energy of at least 0.5 MeV, adetector arrangement arranged to detect radiation after it hasinteracted with or passed through the object, wherein the detectorarrangement comprises a first detector region having a thickness of atleast 2 mm and arranged to detect radiation and a second detector regionhaving a thickness of at least 5 mm and arranged to detect radiationwherein the second detector region is arranged to receive radiation thathas passed through the first detector region.

The scanning system comprises a controller arranged to coordinate timingof irradiation events such that detected information obtained inresponse to an irradiation event is sent from the detector regionsbefore the next event occurs. The first detector region is positionedbetween the object and the second detector region.

The scanning system comprises a controller arranged to determineinformation relating to the object based upon information from the firstand second detector regions relating to the first and second energyprofile radiation.

The controller is arranged to determine information by inputting theinformation from the first and second detector regions relating to thefirst and second energy profile radiation into a least squaresminimization technique to obtain information relating to the object.

The controller is arranged to calculate the ratio, (A/B)₁/(A/B)₂ inorder to the determine information relating to the object based upon thecalculated ratio, wherein A is indicative of the amount of radiationdetected at the first detector region, B is indicative of the amount ofradiation detected at the first detector region, (A/B)₁ is the ratio offirst profile radiation detected at the first detector region relativeto first profile radiation detected at the second detector region, and(A/B)₂ is the ratio of second profile radiation detected at the firstdetector region relative to second profile radiation detected at thesecond detector region.

The scanning system comprises a plurality of detector arrays, eachdetector array comprising a first detector region and a second detectorregion. The scanning system comprises a concentrator and switch arrangedto coherently relay gathered information from the detector regions. Thefirst detector region and the second detector region are configured todetect substantially the same amount of radiation as each other.

The independent claims define aspects of the invention for whichprotection is sought. The dependent claims define preferable inventivefeatures. Any of the features of the dependent claims may be used incombination with the features of other claims or other aspects of theinvention, even if they are not explicitly dependent upon them—this willbe clear to a person skilled in this field.

Where a feature is claimed in one category (e.g. method, system,detector arrangement, etc.) protection is sought for that feature inother categories even if not explicitly claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a flowchart outlining a method according to an embodiment ofthe invention;

FIG. 2 schematically shows a scanning system according to an embodimentof the invention;

FIG. 3 graphically illustrates an output radiation profile from aradiation source used in an embodiment of the present invention;

FIG. 4 schematically illustrates a detector arrangement according to anembodiment of the invention;

FIG. 5 is a graph illustrating different characteristics of high and lowatomic mass objects as seen by the scanning system of an embodiment ofthis invention;

FIG. 6 is a graph illustrating different characteristics of high and lowatomic mass objects as seen by the scanning system of an embodiment ofthis invention;

FIG. 7 is a graph illustrating the change in response relative to theenergy of received radiation;

FIG. 8 is a graph illustrating the change in response relative tointensity of received radiation for high and low atomic mass objects;

FIG. 9 schematically shows a data acquisition system for use with thisinvention;

FIG. 10 illustrates a timing pattern for the data acquisition system ofFIG. 9 in one embodiment; and

FIG. 11 is a representation of overlapping objects which can bedistinguished using this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a method of the invention provides a method10 and system 30 for scanning an object 32. The system 30 comprises aradiation source 36 arranged to irradiate the object 32 with radiation.In this embodiment, the source 36 is a switchable energy linearaccelerator source. Other suitable sources will be apparent to theskilled person. In this embodiment the object 32 moves in the directionof arrow 34 through a scanning zone. The object 32 might be a lorrycarrying cargo for example, driving through a scanning zone, which thesource 36 is arranged to irradiate. In other embodiments, the object 32might be stationary. The radiation source 36 is arranged to operate atleast two different levels. In this example, the source 36 is able tooperate at a high level to provide radiation having a peak energy of atleast 1 MeV and at a low level to provide radiation having a peak energyof at least 0.5 MeV. In this particular example the high level radiationhas a peak value of 6 MeV, and the low level radiation has a peak valueof 3 MeV.

In this context peak value is the energy value at which the highestintensity of radiation is emitted by the source 36.

The scanning system 30 also comprises a detector arrangement 38. Thedetector arrangement is arranged to detect radiation after it hasinteracted with or passed through the object 32 in order to provideinformation to scan the object. The detector arrangement 38 comprises afirst detector 40 and a second detector 42. The first detector 40 has athickness of at least 2 mm. In this embodiment the thickness of thefirst detector is about 15 mm. In other embodiments the thickness may bemore or less and can be tuned as required by a skilled person. Thesecond detector 42 has a thickness of at least 5 mm. In this embodimentthe thickness of the second detector 42 is about 30 mm. Once again, itwill be clear to the skilled person that this detector thickness can bevaried by experimentation in order to tune the detector arrangement 38as required. For example, in some embodiments, the detectors may betuned to detect the same amount of radiation as each other, to providemore efficient signal processing. In this embodiment, referring to FIG.3, the first detector 40 is positioned between the object 32 and thesecond detector 42. In other embodiments, the skilled person mayenvisage a different arrangement. In this particular embodiment, thisarrangement provides a simple geometry in order to achieve the desireddetector configuration such that radiation passing through the firstdetector 40 reaches the second detector 42 after it has interacted withthe object 32. In this embodiment, the detector arrangement 38 is alinear detector array with front 40, A, and rear 42, B, detectors.

The system 30 in its broadest embodiment does not include a movementsensor. In some embodiments, the system 30 includes a movement sensor(not shown). The movement sensor 44 is arranged to measure any one ormore of the position, speed, velocity or acceleration of the object 32,and data gathered using the movement sensor may be used to coordinatetiming of data capture as the object is scanned.

Referring to FIG. 1, the scanning method 10 comprises the step ofproviding 12 a first detector region having a thickness of at least 2mm, and a second detector region having a thickness of at least 5 mm.The method 10 also comprises the steps of irradiating 14 an object to bescanned with radiation having a peak energy value of 1 MeV or more,detecting 16 radiation at the first detector region 40, and thendetecting 18 radiation at the second detector region 42 (the seconddetector region is arranged to receive radiation that has passed throughthe first detector region). The method 10 comprises detecting theradiation after it has interacted with or passed through the object inorder to provide information relating to the object.

The method 10 further comprises irradiating 20 the object with radiationhaving a peak energy value of 0.5 MeV or more (but less than the peakenergy value of the energy irradiated at step 14), and again detecting22 radiation at the first detector region 40, and then detecting 24radiation at the second detector region 42.

In this example the object is scanned with higher energy profileradiation prior to scanning with lower energy profile radiation. Inother embodiments, the object may be scanned with lower energy profileradiation prior to scanning with higher energy profile radiation. In yetfurther embodiments, there may be scanning at more than two differentlevels (each level providing radiation having a different energyprofile).

In this embodiment, the object is irradiated in discrete bursts—in otherembodiments the skilled person will realise that radiation levels can bevaried gradually, or in a combination of bursts and gradual variationand the data collection signal processing should be amended accordingly(this is discussed in more detail below).

In some embodiments, the method 10 comprises sending detectedinformation received in response to a burst from the detector regionsbefore the next burst occurs. This helps to simplify signal processing.

The primary equation that governs X-ray attenuation in matter is

$\begin{matrix}{{I(E)} = {{I_{0}(E)}{\exp \left( {- {\int_{l}{{\mu (E)}{l}}}} \right)}}} & (1)\end{matrix}$

where I(E)=intensity of radiation transmitted through the object atenergy E, Io(E)=intensity of radiation emitted by the source at energyE, μ(E)=linear attenuation coefficient of object at energy E and l=linetaken by the pencil beam of radiation through the object.

The X-ray output from an X-ray linear accelerator is polychromatichaving an energy distribution substantially as shown in FIG. 3. Themaximum X-ray energy (Ep) results from those electron interactions inthe target of the linear accelerator where all of the electron energy istransferred to a single X-ray photon. Typically, less than the fullelectron energy is transferred to a photon resulting in the broad rangeof X-ray energies in the X-ray beam. At low energy, the peaks shown inFIG. 3 are due to fluorescence interactions between the electrons andthe target atoms so resulting in X-rays which are characteristic of thetarget material.

It is customary to use an integrating detector to measure the X-raysignal that is described in equation 1. In this case, the detectedsignal can be written as

$\begin{matrix}{I_{d} = {\int_{0}^{E_{p}}{{I(E)}\left\lbrack {1 - {\exp \left( {- {\int_{s}\ {{\mu_{d}(E)}{s}}}} \right)}} \right\rbrack}}} & (2)\end{matrix}$

where I_(d)=detected signal, μ_(d)(E)=linear attenuation coefficient ofthe detector material at energy E and s=path length of the X-ray beamthrough the detector.

It is therefore clear that I_(d) retains no knowledge of the energydistribution of the incoming X-ray beam, only of the cumulative effectof all X-ray energies.

However, it can also be seen that unless the path through the detectormaterial, s, is very large indeed, some energy will be transmittedthrough the detector (i.e. it will not have a 100% detectionefficiency). Referring to FIG. 4, if a second detector, B, is placed atthe output of the first detector, A, then the energy transmitted throughthe first detector has a chance of being absorbed in the seconddetector. In this case we can write:

$\begin{matrix}{I_{dB} = {\int_{0}^{E_{p}}{{I(E)}{{\exp \left( {- {\int_{s}{{\mu_{dA}(E)}{s}}}} \right)}\left\lbrack {1 - {\exp \left( {- {\int_{t}{{\mu_{dB}(E)}{t}}}} \right)}} \right\rbrack}}}} & (3)\end{matrix}$

where I_(dB)=intensity recorded in detector B, μ_(dA)(E)=linearattenuation coefficient of detector A material at energy E,μ_(dB)(E)=linear attenuation coefficient of detector B material atenergy E and t=path taken by the X-ray beam through detector B.

Inspection of equation 3 shows that the energy spectrum that is incidenton detector B is not the same as the energy spectrum that is incident ondetector A. Therefore, detector A can be thought to have retained someenergy information even though the integrated output alone is notsufficient to tell what this energy information is. The same is true ofdetector B.

In this invention, it is recognised that the measurements that areproduced by detector A and detector B are spatially and temporallycorrelated and that the ratio of the intensity recorded in detector A tothat recorded in detector B will necessarily provide some informationabout the energy distribution of the incident X-ray beam, i.e.

$\begin{matrix}{\frac{I_{dA}}{I_{dB}} = {f\left\{ {I(E)} \right\}}} & (4)\end{matrix}$

where f{ }=function operator.

It can further be seen through inspection of equation (1), that theratio of detector measurements also includes a factor that is due toattenuation in the object.

Three object parameters will affect the ratio of detectors (equation 4)and these are the linear attenuation coefficient of the object, μ(E),the path/taken by the X-ray beam through the object and the energydistribution of the primary beam, Io(E). In this situation, there arethree unknowns and two measurements and therefore it is impossible touniquely determine a value for the three object unknowns.

In another aspect of this invention, it is recognised that if the X-raylinear accelerator can be tuned to produce more than one primary beamdistribution then two pairs of detector results can be collected, onewith a lower energy primary beam distribution, I_(dA)(lo) andI_(dB)(lo), and one with a higher primary beam energy distribution,I_(dA)(hi) and I_(dB)(hi). There are now four measurements with the samethree unknowns and it is therefore possible to determine amathematically unique solution. This solution can be determined using anappropriate numerical technique such as least squares minimisation. Inother embodiments any other similar or suitable numerical technique canbe used as an alternative or in combination.

The present invention is concerned with high energy scanning. At lowenergies (for example most medical scanners), the photo-electric effectis a mechanism by which X-rays interact with matter within objects beingscanned. In contrast, the present invention is concerned with muchlarger X-ray source energies—namely, lower energy primary beamdistribution mentioned above has a peak value of 500 keV or above (andthe higher energy beam has a value higher than this). The predominantmechanism governing interactions of radiation within matter at theseenergies is Compton scattering.

The attenuation in matter of X-rays affected by the photo-electriceffect shows a dependence proportional to Z⁴ (where Z=atomic number). Incontrast, Compton scattering produces a Z¹ dependence. Some Comptonscattering is also present at low energies.

The detector regions of the present invention are configured such thatin the front detector, there is an approximately Z⁴ dependence arisingfrom a combination of the photo-electric and Compton scattering effects.The second, rear detector has a Z¹ dependence. As a result there aresignificantly different considerations compared to low energy X-rayscanning, due to the different physical laws governing the interactionof matter. The inventor has realised that for high energy X-ray scanningapplications, the front and rear detectors in the claimed arrangementare governed by different physical laws with regards to theirinteraction with high energy radiation. As a result of the differentphysical relationships, different detector arrangements are required,relative to low energy X-ray scanners. Accordingly, a first detector isspecified as being at least 2 mm thick, whilst the second detector isspecified as being at least 5 mm thick. Also, different signalprocessing is required to account for the combination of thephoto-electric effect and Compton scattering occurring at the firstdetector, and the predominantly Compton scattering effect at the seconddetector. As a result conventional cargo scanners do not have a dualdetector region arrangement as specified in this invention.

The detector arrangement for use in a scanning system of this type (i.e.the system comprises a radiation source arranged to irradiate an objectto be scanned, wherein the detector arrangement is arranged to detectradiation after it has interacted with or passed through the object inorder to scan the object) may be a stacked detector, wherein thedetector arrangement comprises a first detector region arranged todetect radiation and a second detector region arranged to detectradiation wherein the second detector region is arranged to receiveradiation that has passed through the first detector region. In thisexample the first detector region is positioned between the object to bescanned and the second detector region. The first detector region andthe second detector region are configured to detect a predeterminedamount of radiation relative to each other—in this example, the firstdetector region and the second detector region are configured to detectsubstantially the same amount of radiation as each other—in this examplethis is achieved by configuring the lengths s, t of the detectors A, B.

Both or each of the first detector and the second detector may comprisea linear detector array.

An example of the data that can be recorded using a system with stackeddetectors as exemplified in this invention (and as shown in FIG. 4) isgiven in FIGS. 5 to 8. In these figures, I is the total integratedintensity of radiation detected, i.e. the sum of the intensity at thefirst detector, A, and the second detector, B. F/R is a measure of theratio of intensity of radiation detected at the front and reardetectors. L/H is a measure of the ratio of intensity of radiationdetected at low and high source energy profile.

At lower energies, the front detector absorbs most of the radiationwhich reaches it. As a result there is a good distinction relative tothe absorption at the rear detector between high Z and low Z objects,where Z=atomic mass. Therefore the ratio, F/R provides particularlyuseful information at low energies.

At higher energies, the L/H ratio provides good distinction between highZ and low Z objects. Therefore the ratio, L/H provides particularlyuseful information at high energies.

In combination these two ratios help to provide comprehensiveinformation across the energy spectrum.

FIG. 8 shows the percentage difference between the two curves in FIGS. 5and 6. As a guide the difference between the intensity ratios at low andhigh energies can be as large as 10 percent. Given that the noise floorin a good quality detection system should be on the order of 10 partsper million, a several percent change in intensity ratio is verymeasureable.

In one embodiment, a suitable data acquisition system 90 for use withthe scanning system is shown in FIG. 9. Here a pulsed X-ray accelerator92 has two inputs, Trigger and Energy. X-rays from the accelerator 92pass through the object under inspection and intercept sensor arrays 94that have front and rear sensor elements. The analogue signal isintegrated and converted to a digital form prior to transmission to aset of concentrator cards 96 which format Ethernet packets that containthe digitised sensor data. These Ethernet packets are passed from eachConcentrator card back through an Ethernet Switch 98 to a controllingcomputer 100 where they are formatted into lines in an image which arethen displayed on a human readable monitor 102. Each line in the imagecorresponds to one accelerator pulse worth of sensor data. Of course,other data acquisition system architectures are quite workable and willbe apparent to the skilled person, and FIG. 9 is presented as an exampleof good practice in data acquisition design.

FIG. 10 presents an example of a timing diagram for acquisition ofquad-energy X-ray data. A trigger pulse trigger switching of the energylevel of the radiation source between its high and low levels. Detectionevents are integrated at each detector co-ordinated in time with thehigh and low energy states, and readout from each detector occurs priorto the next integration event.

In some embodiments it is advantageous to use an offset staggered rowdetector to improve scanning speed, to increase detection efficiency andto provide improved spatial correlation between the high and low energyX-ray measurements. This may be done to achieve Nyquist sampling ratesfor example.

In some embodiments, it can be advantageous to utilise non-periodicpulse sequences of radiation from the source in order to assist inreducing dose rates and to provide superior object penetrationperformance.

In a further aspect of this invention, it is observed that the analysisprovided in equations 1 through 4 above relates to a single homogeneousobject. In a real object, there are often multiple objects which mayoverlap in the image. An example of overlapping objects is shown in FIG.11. Here a first object 110 is partially overlapped by a second object112. In each case, the overall shape of the objects is visible to thehuman eye, even in the overlap region 114. In this invention it isclaimed that automated image processing methods can be used to segmentthe projected quad-energy X-ray image to resolve the materialscharacteristics of the region to the left of the first object 110 andthe region to the right of the second object 112. This information aboutthe objects 110 and 112 can then be used to analyse the overlapping area114. Knowing the beam quality, thickness and attenuation coefficient ofthe first object 110 and the thickness and attenuation coefficient ofthe second object 112, it is possible to calculate the expectedintensity that should be detected in the overlap region using thefollowing equation:

$\begin{matrix}{{I(E)} = {{I_{0}(E)}{\exp \left( {- \left\{ {{\int_{0}^{t_{1}}{{\mu_{1}(E)}{l}}} + {\int_{0}^{t_{2}}{{\mu_{2}(E)}{l}}}} \right\}} \right)}}} & (5)\end{matrix}$

where t₁=thickness of the first object 110, μ₁ (E)=attenuationcoefficient of the first object 110 at energy E, t₂=thickness of thesecond object 112 and μ₂(E)=attenuation coefficient of the second object112 at energy E. Note that the detected values, I_(dA)(lo), I_(dB)(lo),I_(dA)(hi) and I_(dB)(hi) are then determined through equations 3 and 4.The measured values can then be compared to the associated calculatedvalues to ensure that noting else is present in the overlap region 114.

It is noted that the techniques discussed here can be extrapolated tofurther more complex situations. For example, an X-ray source could bedeveloped to operate at more than two beam energies and more than twodetector layers could be assembled to give finer still sampling of theenergy distribution of the transmitted X-ray signals. The data analysismethods are the same, but there are further measurements of the samenumber of unknowns and it is therefore in principle possible to generatea better determined solution.

In an alternative embodiment, metal filter layers could be interposedbetween the detector elements in order to further shape the X-rayspectrum. This method is not recommended since no signal is recordedfrom the metal filter layer and the net result is a higher dose imagethan when an active detector is used as a filter for the equivalentpurpose.

Various modifications may be made to this invention without departingfrom its scope (as defined by the claims). The high and low energyx-rays may be sent in a different order, e.g. low energy, then highenergy.

Different ratios may be calculated with the four elements ofinformation. The ratio described in the above example:(A/B)_(lo)/(A/B)_(hi) is particularly useful since it takes away theneed for calibration of the detectors. Other unique ratios which can beused are A_(hi)/A_(lo), A_(hi)/B_(lo), A_(lo)/B_(hi), A_(hi)/B_(hi),A_(lo)/B_(lo). These are unique ratios—B_(hi)/A_(lo) can be used butoffers no advantage above A_(lo)/B_(hi) since it is merely itsinverse—similarly for other ratio examples.

In some embodiments more than two detector regions may be provided andmore than two radiation energy levels may be used for irradiation of theobject. For example instead of two, three or four or five or six or anyother suitable number of energy levels may be used.

Exactly the same detector array principle can be used with other imagingprobes including thermal neutrons and fast neutrons which can provideadditional diagnostic benefit.

It will be clear to the skilled person that different peak values forthe lower energy profile and/or the higher energy profile may be usedwithin the bounds specified by the claims. For the example the lowerprofile peak energy value may be 4 MeV, and the higher profile peakenergy value may be 7 or 8 MeV.

1. A scanning method for scanning an object comprising: providing a first detector region having a thickness of at least 2 mm and a second detector region having a thickness of at least 5 mm wherein the second detector region is arranged to receive radiation that has passed through the first detector region; irradiating the object with radiation having a peak energy of at least 1 MeV; detecting the first profile radiation after it has interacted with or passed through the object in order to provide information relating to the object, wherein detecting the first profile radiation comprises: detecting the first profile radiation at the first detector region; receiving the first profile radiation that has passed through the first detector region at the second detector region; detecting the first profile radiation at the second detector region; the scanning method further comprising: irradiating the object with radiation having a second energy profile, relatively lower than the first energy profile, and having a peak energy of at least 0.5 MeV; detecting the second profile radiation after it has interacted with or passed through the object in order to provide information relating to the object, wherein detecting the second profile radiation comprises: detecting the second profile radiation at the first detector region; receiving the second profile radiation that has passed through the first detector region at the second detector region; detecting the second profile radiation at the second detector region.
 2. The method of claim 1 comprising positioning the first detector region between the object and the second detector region.
 3. The method of claim 1 comprising determining information relating to the object based upon information from the first and second detector regions relating to the first and second energy profile radiation.
 4. The method of claim 3 comprising determining information by inputting the information from the first and second detector regions relating to the first and second energy profile radiation into a least squares minimization technique to obtain information relating to the object.
 5. The method of claim 3 comprising calculating the ratio, (A/B)₁/(A/B)₂ in order to the determine information relating to the object based upon the calculated ratio, wherein A is indicative of the amount of radiation detected at the first detector region, B is indicative of the amount of radiation detected at the first detector region, (A/B)₁ is the ratio of first profile radiation detected at the first detector region relative to first profile radiation detected at the second detector region, and (A/B)₂ is the ratio of second profile radiation detected at the first detector region relative to second profile radiation detected at the second detector region.
 6. The method of claim 1 comprising irradiating and detecting the first profile radiation before the second profile radiation, or vice versa.
 7. The method of claim 1 wherein irradiating the object comprises irradiating the object in discrete bursts.
 8. The method of claim 7 comprising sending detected information received in response to a burst from the detector regions before the next burst occurs.
 9. The method of claim 7 wherein the low energy profile radiation comprises 3 MeV x-ray radiation and the high energy profile radiation comprises 6 MeV x-ray radiation.
 10. The method of claim 1 comprising configuring the first detector region and the second detector region to detect a predetermined amount of radiation relative to each other.
 11. The method of claim 10 comprising configuring the first detector region and the second detector region to detect substantially the same amount of radiation as each other.
 12. The method of claim 10 comprising configuring any one or more of size, shape or material of the or each detector region so that the first detector region and the second detector region detect the predetermined amount of radiation relative to each other.
 13. The method of claim 1 comprising providing a first detector including the first detector region and a second detector including the second detector region.
 14. The method of claim 1 comprising irradiating the object with radiation at more than two energy profiles, such as at three energy profiles or four energy profiles or five energy profiles or six energy profiles or seven energy profiles.
 15. A method of scanning overlapping objects comprising using the method of claim 1 to determine information relating to each overlapping object in a region of the object which does not overlap another object and using the determined information to calculate a reference detection value or values relating to a value or values expected to be detected in the region in which the objects overlap in the absence of further objects that are not present outside the overlapping region and using the method of any preceding claim to ascertain information relating to the region in which the objects overlap and comparing the ascertained information to the expected values to determine whether an additional object is present within the region in which the objects overlap.
 16. A scanning system for scanning an object comprising: a variable energy level radiation source arranged to irradiate an object with radiation having a plurality of different energy profiles including a first energy profile having a peak energy of at least 1 MeV and a second relatively lower energy profile having a peak energy of at least 0.5 MeV, a detector arrangement arranged to detect radiation after it has interacted with or passed through the object, wherein the detector arrangement comprises a first detector region having a thickness of at least 2 mm and arranged to detect radiation and a second detector region having a thickness of at least 5 mm and arranged to detect radiation wherein the second detector region is arranged to receive radiation that has passed through the first detector region.
 17. The scanning system of claim 16 comprising: a controller arranged to coordinate timing of irradiation events such that detected information obtained in response to an irradiation event is sent from the detector regions before the next event occurs.
 18. The scanning system of claim 16 wherein the first detector region is positioned between the object and the second detector region.
 19. The scanning system of claim 16 comprising a controller arranged to determine information relating to the object based upon information from the first and second detector regions relating to the first and second energy profile radiation.
 20. The scanning system of claim 19 wherein the controller is arranged to determine information by inputting the information from the first and second detector regions relating to the first and second energy profile radiation into a least squares minimization technique to obtain information relating to the object.
 21. The scanning system of claim 19 wherein the controller is arranged to calculate the ratio, (A/B)₁/(A/B)₂ in order to the determine information relating to the object based upon the calculated ratio, wherein A is indicative of the amount of radiation detected at the first detector region, B is indicative of the amount of radiation detected at the first detector region, (A/B)₁ is the ratio of first profile radiation detected at the first detector region relative to first profile radiation detected at the second detector region, and (A/B)₂ is the ratio of second profile radiation detected at the first detector region relative to second profile radiation detected at the second detector region.
 22. The scanning system of claim 16 comprising a plurality of detector arrays, each detector array comprising a first detector region and a second detector region.
 23. The scanning system of claim 22 comprising a concentrator and switch arranged to coherently relay gathered information from the detector regions.
 24. The scanning system of claim 16 wherein the first detector region and the second detector region are configured to detect substantially the same amount of radiation as each other. 